Project Summary (Significance) Multiple lines of evidence have implicated mGluR5 signaling for the pathophysiology and treatment of schizophrenia; yet possible dysregulation of mGluR5 signaling and their pathophysiologic roles are presently unknown. Recently, we found direct evidence for reduced mGluR5 signaling in the postmortem dorsolateral prefrontal cortex (DLPFC) of patients with schizophrenia. mGluR5 is physically connected with NMDA receptor (GluN) complexes in the postsynaptic density (PSD) and these two pathways facilitate the activity of each other. (Preliminary Studies) We examined the agonist-induced activation of mGluR5 signaling in the postmortem dorsolateral prefrontal cortex (DLPFC) derived from 17 matched pairs of patients and controls. We found a striking reduction in three pathways downstream to mGluR5 receptor activation; namely, Gq/11 activation and recruitment of PI3K and scaffolding proteins compared to controls (P<0.01). This dysregulation was accompanied by alterations in phosphorylation of mGluR5, important for receptor desensitization, and in mGluR5 association with RGS4, Preso 1, norbin and tamalin, which are critical for surface expression and clustering. Importantly, we find evidence supporting disruption of reciprocal facilitation between mGluR5 and GluN signaling in schizophrenia and thus hypofunction of one can further compromise the other. (Scientific Premise) Schizophrenia is associated with mGluR5 signaling hypoactivity mediated by altered mGluR5 phosphorylation and protein ? protein interactions (PrPrIs) in the receptor complexes. mGluR5 hypofunction can contribute to GluN signaling hypoactivity and vice versa via disruption of reciprocal facilitation. PrPrIs in the interactome may be a point of convergence for etiologic factors. (Specific Aims) To further characterize mGluR5 hypoactivity and its interplay with GluN signaling in schizophrenia, we will examine molecular underpinnings for mGluR5 hypoactivity on GluN signaling and vice versa in the DLPFC of patients compared to controls. (Aim 1). Our preliminary data and recent studies point to PrPrIs as substrates upon which various molecular alterations converge and precipitate glutamatergic dysregulation. Combining discovery and quantitative proteomics, we will analyze PrPrI alterations in the mGluR5-GluN interactome in patients at the basal level and in response to receptor activation (Aim 2). PrPrI alterations in the interactome can be traced to their etiologic underpinnings at the genomic, transcriptomic and proteomic levels. Cutting edge systems biology algorithms permit us to leverage largest genomics data sets presently available and impute DLPFC transcriptomics data of the same number of subjects. Genetic variants, imputed transcriptomic results, along with quantitative PSD proteomics results will be projected onto the interactome using heat diffusion based algorithms (Aim 3). This will identify etiologically significant and potentially targetable subnetworks in the interactome. (Impact) We will establish mGluR5 hypofunction as integral to glutamatergic dysregulation in schizophrenia and identify etiologic underpinnings and potential points of intervention.